In the oil and gas industry, it is desirable to obtain data from a wellbore. Several real time data systems have been proposed. One involves the use of a physical cable such as an electrical conductor or a fiber optic cable that is secured to the tubular body. The cable may be secured to either the inner or the outer diameter of the pipe. The cable provides a hard wire connection that allows for real-time transmission of data and the immediate evaluation of subsurface conditions. Further, these cables allow for high data transmission rates and the delivery of electrical power directly to downhole sensors.
It has been proposed to place a physical cable along the outside of a casing string during well completion. However, this can be difficult as the placement of wires along a pipe string requires that thousands of feet of cable be carefully unspooled and fed during pipe connection and run-in. Further, the use of hard wires in a well completion requires the installation of a specially-designed well head that includes through-openings for the wires.
Various wireless technologies have been proposed or developed for downhole communications. Such technologies are referred to in the industry as telemetry. Several examples exist where the installation of wires may be either technically difficult or economically impractical. The use of radio transmission may also be impractical or unavailable in cases where radio-activated blasting is occurring, or where the attenuation of radio waves near the tubular body is significant.
The use of acoustic telemetry has also been suggested. Acoustic telemetry employs an acoustic signal generated at or near the bottom hole assembly or bottom of a pipe string. The signal is transmitted through the wellbore pipe, meaning that the pipe becomes the carrier medium for sound waves. Transmitted sound waves are detected by a receiver and converted to electrical signals for analysis.
Faster data transmission rates with some level of clarity have been accomplished using electromagnetic (EM) telemetry. EM telemetry employs electromagnetic waves, or alternating current magnetic fields, to jump across pipe joints. In practice, a specially-milled drill pipe may be provided that has a conductor wire machined along an inner diameter. The conductor wire transmits signals to an induction coil at the end of the pipe. The induction coil, in turn, transmits an EM signal to another induction coil, which sends that signal through the conductor wire in the next pipe. Thus, each threaded connection provides a pair of specially milled pipe ends for EM communication.
While high data transmission rates can be accomplished using RF signals in a downhole environment, the transmission range is typically limited to a few meters. This, in turn, requires the use of numerous repeaters.
A downhole network is often powered by batteries that can be energy-limited because of the constraints of space and cost. To extend the useful life of the network requires saving battery power. However, there is lack of methods to wake the nodes up when the data acquisition is required. As a result, the life of the network is limited to months, not years.
Accordingly, a need exists for a high speed wireless transmission system for use in a wellbore that offers extended battery life.